JAKARTA, Indonesia (CNN) -- A strong earthquake with a preliminary magnitude of 6.4 struck off the southwestern coast of Sumatra Friday in the same area shaken by a major 8.4-magnitude temblor that killed 13 people earlier in the week.

A woman salvages items from her newly built house at Air Besi in North Bengkulu Thursday.

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Wednesday's quake generated a series of aftershocks, including two major ones early Thursday measuring 7.8 and 8.1, said David Applegate, senior science adviser at the U.S. Geological Survey.

"It's been an incredible number of years for Indonesia and particularly for Sumatra" in terms of earthquakes, Applegate said on CNN's "American Morning" on Thursday.

"What we have here is a subduction zone, where one of the Earth's plates is moving down beneath the other," he said. "In this case, the Indian Ocean and the Australian Plate are moving beneath the Eurasian Plate.

"In this kind of a situation you're going to get earthquakes as the strain builds up, but what we're seeing now is almost every segment of this plate has ruptured just in the last several years," Applegate said.

"In each case, it relieves pressure in one area but then that increases the pressure somewhere else. And so, for example, what we saw yesterday was the magnitude-8.4 quake ruptured to the north along this boundary. This 7.8 was at the northern end of that."

People in the Indian Ocean region have been extremely skittish about the possibility of earthquake-induced tsunamis since December 2004, when gigantic waves triggered by a 9.1-magnitude quake killed more than 200,000 people in seven countries.

Since the December 2004 tsunami, almost 8,000 people have died in 15 earthquakes of magnitude 6.3 or higher, according to the USGS.E-mail to a friend